The present invention relates to elastomeric lip seals of the type including a protruding annular seal lip which is radially loaded to make constant pressure contact with a dynamic surface to which the lip is sealed. Typical applications are rod and piston seals for forming good fluid seals at low friction and high pressures. One lip seal of this type has an elastomeric jacket of generally U-shaped cross-section and a stainless steel spring energizer molded into the elastomeric jacket of the seal to form a one-piece seal. The spring provides radial loading to energize either the ID or OD lip of the elastomeric seal for its contact with the dynamic surface.
One prior art seal of this type is known as the ES seal and is sold by The Fluorocarbon Company, the assignee of this application. The ES seal has an elastomeric sealing jacket which provides excellent leakage control, particularly in single-acting piston/rod applications. The spring sustains radial loading on the seal lips, particularly at low temperatures, but also at high temperatures, where the natural resilience of the elastomer normally fades. Use of the spring energizer in the seal not only maintains continuous loading on the entire periphery of the seal lip, but also allows a high degree of flexibility and mobility for easy installation of the seal into many closed grooves.
Prior art rubber U-cup packings or seals also may have no spring energizer, in which case the rubber jacket can have thick "legs" to provide enough spring energy to the seal lips to form a reliable fluid seal. However, at both high and low temperature extremes, the legs lose some resilience, and leakage control is not as effective.
Rubber seals tend to suffer from shrinkage of the rubber at low temperatures experienced by aircraft (as low as -65.degree. F.) which detracts from otherwise good leakage control. Fluorocarbon-type elastomers are especially susceptible to this problem.
The ES seal overcomes these problems by its use of the spring energizer, which provides an almost constant load on the seal lips, while extending both upper and lower effective operating temperatures of all elastomeric materials used for the seal.
The elastomeric seal legs tend to make the ES seal difficult to move against the dynamic surface (rod or bore) of a hydraulic cylinder under certain conditions. For instance, break-out friction forces are experienced when the seal has been standing for some time, either pressurized or unpressurized. This phenomenon is known as "stiction" or "stick-slip" which also occurs at low velocities. It is caused by lubricating film under the seal lip tending to be squeezed out due to the load on the seal lip. This effect is more pronounced with a soft seal material which conforms better to the microcontours of the hardware surface.
When high pressures are applied to the ES seal, a separate back-up ring is usually necessary to help resist "extrusion" of the heel of the seal. Extrusion is an unwanted deformation of the elastomer in which the elastomer is squeezed out of the annular groove in which it is seated. These back-up rings which resist extrusion are commonly made from polytetrafluoroethylene (PTFE), or filled PTFE, but high-modulus engineered plastics may also be used.
Another seal used in applications similar to the ES seal is sold by W. S. Shamban & Co. and known as the Hatseal. This seal comprises a Teflon slipper which has a back-up ring section. The seal has an elastomer contact which is claimed to reduce the amount of contact as pressure increases, by the pressure deforming the rubber so that the axial length of the seal is decreased. Since the Teflon (trademark of duPont) part maintains its dimensions, the rubber contact on the dynamic surface decreases. This causes lower wear on the elastomeric sealing lip, which also provides for lower friction. The Teflon component has lower friction than rubber when under pressure. The dynamic leg of the Teflon part has a back-up ring section to prevent extrusion of the elastomer. The shape of the elastomer is such that all radial vectored forces basically equalize to provide equilibrium and so that radial force with increased pressure is maintained at a minimum.
There are several disadvantages of the Hatseal. For particularly small sizes, assembly of the Teflon part is difficult because it occupies full groove depth. A subsequent design known as Hatseal II uses two Teflon parts for the back-up ring component. This design has disadvantages from tolerance stack-up, from practical manufacturing tolerances, and greater risk of misassembly due to the large number of parts. Also, with the groove on maximum depth and the Teflon parts on minimum tolerances, the resulting gap may lead to the elastomer being nibbled with continuing reversals of pressure, finally resulting in seal failure. Also, because the elastomer wipes dry on the dynamic surface, the Teflon leg suffers from a lack of lubrication and subsequent high wear reduces it to a thin, sharp sliver of Teflon. Due to movement of the rubber against the Teflon with increasing pressure, the rubber lip can be cut off by the sharp Teflon piece and result in failure of the seal.
Low temperature affects the Hatseal elastomer by shrinkage, causing loss of radial squeeze against the Teflon part, some loss of load on the rubber contact seal lip, and also some loss of resilience and speed of response. This series of effects leads to significant loss of control. The effects vary with the rubber used but are most prevalent with fluorocarbon rubber such as Viton (trademark of duPont) materials. This limits these materials to a lowest usable temperature of about -15.degree. F.
A prior art seal designed and developed by Boeing Corporation is known as the Footseal. A variation of this seal, known as Footseal II, is sold by W.S. Shamban & Co. These seals use the same groove dimensions and basically have the same geometries, with Footseal II having some axial and more radial squeeze as an improvement over the original Footseal design. These seals consist of an L-shaped Teflon-based part energized by an O-ring. Advantages of the Footseal are low pressure driving the Teflon part toward the dynamic (rod) surface by virtue of the edge profile at the low-pressure gland wall, to provide more effective sealing. The ramp angle where the O-ring energizes the dynamic Teflon leg results in the O-ring's providing greater squeeze under pressurized conditions to provide more effective sealing. The back-up ring part of the assembly provides good resistance to extrusion to protect the O-ring.
The Footseal also has a number of disadvantages. Because of the large radial depth of the Teflon part, it is not easy to install the seal in a closed gland, particularly in small diameters. This necessitates using an open gland, which usually is more expensive, occupies more room, and has a greater weight penalty. Friction under pressure is high due to increased squeeze from the O-ring, and the cam-action of the angled wall of the gland which vectors the axial pressure load into a load perpendicular to the rod surface, thereby generating higher friction loads. The seal also suffers from loss of squeeze, resilience, and response at low temperatures due to low-temperature effects on the O-ring rubber used. A fluorocarbon rubber such as Viton is particularly affected negatively and cannot be used at temperatures below about -15.degree. F. Further, some leakage is experienced with seals having Teflon contact only.
Thus, there is a need for an elastomeric lip seal which provides a reliable fluid seal and has good long-term wear properties, especially under temperature extremes and low lubricity conditions. Low dynamic friction, low break-out force, resistance to compression set, and resistance to extrusion of the elastomer under high operating pressures are sought-after properties that enhance seal performance. In addition, ease of installation of the seal is important.